Waterborne inks, coatings, and adhesives are commonly applied to a variety of substrates including plastic films, paper, metal, concrete and board stock. They are an attractive and environmentally friendly option to solvent-borne systems which tend to have a substantial content of volatile organic compounds (VOCs). However, one deficiency of waterborne systems is the lack of adhesion and permanent resistance to water and other chemicals such as isopropanol, oils, greases, solvents, etc. One reason for this is the nature of the composition of the ink or coating system. Typically, waterborne systems include emulsion polymers supported by surfactants. The presence of these surfactants (necessary for forming a stable emulsion) in small quantities deteriorates the resistance properties of the final dried ink/coating. Alternatively, the emulsion polymerization may be supported using acid functional styrene-acrylic protective polymer colloids in place of surfactants. These improve resistance properties and also provide additional benefits such as resolubility (the ability of the press to recover print after the press has stopped) of the inks on the press. Unfortunately, for certain applications such as printing inks on films, the resistance properties are still not sufficient when using traditional protective colloids with high acid content.
Low molecular weight acid functional acrylic polymers are often used as protective colloids in emulsion polymerization. These materials can be conveniently made in a high-temperature, continuous polymerization process, typically above 175° C. Usually, styrenic monomers and/or acrylates and methacrylates are used as comonomers. The incorporation of styrene yields hard and cost effective polymers, however, this also requires high levels of acid monomer incorporation so as to be dispersible in water with a suitable base, such as ammonia, for example.
To address the above issues, additional cross-linkers, such as aziridines, carbodiimides, zinc compounds or zirconium compounds, may be added to react with the acid groups. However, such approaches also have drawbacks, including cost to the final product and toxicity or stability issues related to the cross-linkers. Thus, there is a need for a stable waterborne emulsion system that can be cross-linked with a cost effective amount of cross-linker to give acceptable resistance properties, while maintaining important properties such as ink resolubility.